Phosphate conversion coating of aluminum, zinc or iron

ABSTRACT

A zinc phosphate coating process for coating a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least 1 gram per liter of phosphate ion 3as PO4), (c) at least about 1 gram per liter of nitrate ion (calculated as NO3), (d) between about 0.006 gram per liter and about 1 gram per liter of nitrite ion (calculated as NO2), (e) between about 0.025 gram per liter and about 2.5 grams per liter of fluoride added as a combination of fluorides and bifluorides of sodium and potassium, such that the molar ratio of potassium to sodium is about 2:1 and the ratio of fluoride to bifluoride is about 1:1.

United States Patent Inventors Ferdinand Phillip Heller Philadelphia; Mark Allen Kuehner, North Hills; Lester Steinbrecher, Southampton, all of Pa. Appl. No. 775,517 Filed Nov. 13, 1968 Patented Nov. 9, 1971 Assignee Amchem Products, Inc.

Lower Gwynedd Township, Montgomery County, Pa.

PHOSPHATE CONVERSION COATING OF ALUMINUM, ZINC OR IRON 7 Claims, No Drawings 0.8. CI. l48/6.l5, 148/627 Int. Cl C23f 7/10,

[50] Field of Search l48/6.l5 Z,

ABSTRACT: A zinc phosphate coating process for coating a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least I gram per liter of phosphate ion 3as PO (c) at least about 1 gram per liter of nitrate ion (calculated as NO;,), (d) between about 0.006 gram per liter and about 1 gram per liter of nitrite ion (calculated as N0 (e) between about 0.025 gram per liter and about 2.5 grams per liter of fluoride added as a combination of fluorides and bifluorides of sodium and potassium, such that the molar ratio of potassium to sodium is about 2: l and the ratio of fluoride to bifluoride is about 1:1.

PHOSPHATE CONVERSION COATING OF ALUMINUM, ZINC OR 'IRON BACKGROUND OF THE INVENTION It is known to form zinc phosphate conversion coatings on the surface of aluminum and its alloys as well as on other metal surfaces including iron and galvanized. Possibly, the first disclosure of such a process was in U.S.'Pat. 2,312,855, which described the use of a solution containing a dihydrogen I phosphate, an oxidizing agent (preferably, sodium nitrate) and a complex fluoride, for example, sodium 'silicofluoride. Another example of a solution formulated to deposit a zinc phosphate conversion coating on aluminum and its alloys is set forth in U.S. Pat. No. 2,487,137 which discloses solutions containing as essential ingredients, a dihydrogen phosphate, an oxidizing agent, ferrous ion and fluoride ion provided exclusively by hydrofluoric acid and its salts. These solutions are adjusted to exhibit a total acidity" and free acidity" within defined pointage ranges, these properties referring to the acidity of the solution as measured by titrating a predetermined sample against standard alkali using, respectively phenolphthaleir (about pH 9 and brom-phenol blue (about pH 4 as the end-point indicators.

These prior art solutions suffer from two main drawbacks. The first is that while it is possible to set up a solution to coat aluminum after this fashion, it is exceedingly difficult to maintain such a solution in operating condition, owing to the hydrolysis of zinc phosphate to insoluble tertiary zinc phosphate with attendant heavy sludging, zinc depletion and free acid build-up The second, which is related to the first, is that in the formation of phosphate coatings on the surfaces of aluminum and its alloys, free aluminum enters the solution, the presence of which is disadvantageous because it interferes with and even prevents the subsequent formation of the phosphate coating.

Of all the known prior art phosphate coating solutions only one seems to have been commercialized to any extent for use with more than one substrate, i.e. steel, zinc or aluminum, and this is the subject of US. Pat. No. 2,500,673. In that disclosure use is made of a solution containing as essential ingredients, zinc dihydrogen phosphate, nitric acid as oxidizing agent and a fluoborate, the latter material is formed in situ by the separate addition of sodium bifluoride and excess boric acid so that free boric acid is also present. The solutions are replenished in operation by adding a neutralizing sodium compound and an acidic fluoride in amounts sufficient of form Na AlF (cryolite) with a part of the dissolved aluminum in the coating solution. The cryolite precipitates out thereby removing some of the excess aluminum from the solution.

In practice this process is exceedingly cumbersome owing to the difficulty in controlling and maintaining the composition of the coating solution. For example, to replenish the solution, it is necessary to add from separate sources,

a. zinc phosphate replenishing solution,

b. sodium bifluoride,

c. boric acid, and

d. sodium carbonate.

The make-up solution (a) is added to replenish the zinc and the phosphate ions depleted from the coating solution by the fonnation of the zinc phosphate conversion coating. Sodium bifluoride (b) is added as needed to complex and precipitate the dissolved aluminum formed during the coating process. The boric acid (c) is required to complex the excess fluoride. And sodium carbonate (d) is necessary to neutralize the excessive acidity caused by the addition of the acidic sodium bifluoride. A further difficulty is that the excess aluminum is precipitated from solution at a relatively slow rate owing to the nature of the Na AlF precipitate so that the proportion of free aluminum ions remaining dissolved in the coating solution must be capable of removing free aluminum from the solution as fast as it enters, since the free aluminum ions have a poisoning effect on the formation of the phosphate coating.

It is therefore an object of this invention to provide solutions and processes commercially suited for the formation of zinc phosphate conversion coatings on aluminum and alloys thereof, as well as on zinc and iron and their alloys and to'provide for maintaining the solutions capable of continually depositing a zinc phosphate coating of high quality by the addition of replenishers whereby the coating solution is maintained in optimum operating condition without introducing undesired contaminants and rendering innocuous such solution contaminants as may be formed.

BRIEF DESCRIPTION OF THE INVENTION According to the present invention, there are provided aqueous acidic solutions for the deposition of zinc phosphate conversion coatings on substrates comprised of aluminum or its alloys and zinc or iron, and particularly such solutions which will function even when a substantial portion of the total surface area treated is aluminum or an alloy of aluminum. The coating solutions of this invention comprise essentially zinc, phosphate, nitrate, nitrite and fluoride ions, so adjusted as to exhibit a certain free acidity" and total acidity" as will be more fully set forth hereinbelow.

Solutions such as these are the so-called coating phosphate solutions, wherein the cation of the phosphate employed (in this case,zinc) is actually incorporated into the coating formed, and can be maintained by adding to the solution sufficient amounts of a concentrate of zinc ion, phosphate ion, and nitrate ion to replenish the zinc and phosphate removed by the coating and maintain the free acidity and total acidity within their respectivelydesired ranges. Maintenance of the acid concentration sufficient to produce the desired coating while avoiding excess phosphate which causes sludging is accomplished by adding nitric acid. Sufficient nitrite ion is added, preferably for economic considerations in the form of sodium nitrite, to avoid the accumulation of iron in the coating bath when processing steel and to consume hydrogen when processing aluminum. Fluoride ion is added as alkali metal bifluorides of sodium and potassium whereby the undesirable aluminum ion is readily removed from solution as a dense precipitate of K,NaAlF,,.

Other objects and advantages of the present invention will become more apparent from the detailed description and the examples which follow setting forth details of the compositions and processes according to this invention.

DETAILED DESCRIPTION In its broad aspect this invention pertains to; a process for coating metal surfaces with a phosphate coating, coating solutions and premixes used in such process, replenishing solutions and premixes for maintaining the composition of the coating solution within optimum parameters and metal products coated according to the process of this invention. In particular the invention relates to coating compositions for use in treating either aluminum, iron, or zinc surfaces, which compositions comprise solutions containing as essential ingrcdients zinc ion, phosphate ion, nitrite ion, nitrate ion, fluoride ion and both sodium and potassium ions.

Probably one of the most serious defects of the commercialized prior art phosphating solutions for iron, zinc and aluminum is their tendency to very high sludging due to hydrolysis in the bath producing large quantities of insoluble tertiary zinc phosphate, Not only does this sludge create problems in the actual operation of the bath-clogging pumps, contaminating coatings and so on-but it also represents a large proportion of the zinc used up" in the bath, zinc which was intended for the coating itself.

According to the present discovery a significant portion of the phosphate content of the bath is replaced by nitrate thereby substantially, and in most cases completely preventing the formation of a zinc sludge. Furthermore, by regulating the absolute and relative quantities of zinc, phosphate and nitrate, according to the process aspects of this invention the quality of the coatings fonned is as high as, if not higher than, any coatings hitherto attainable using a bath specifically adapted to coat either iron, zinc or aluminum individually. More importantly the addition of fluorides in the manner of this invention allows the bath to be operated successively or simultaneously with either of these metals utilizing the same bath composition. it will, therefore, be appreciated that the coating compositions as more specifically described hereinbelow, particularly as regards the nitrate, nitrite and fluoride components are especially important to the success of the process.

Thus in a more particular aspect this invention provides an aqueous acidic zinc phosphate solution adapted for the formation of phosphate conversion coatings on the surfaces of iron, zinc and/or aluminum, as well as alloys thereof, which in a preferred embodiment had the following characteristics:

1 at least about 0.7 grams/liter of zinc;

2. at least about i gram/liter of phosphate (calculated as P04);

3. at least about 1 gram/liter of nitrate (calculated as N 4. from about 0.006 to about 1 gram/liter of nitrite (calculated as NO,);

5. from about 0.025 to about 2.5 grams/liter of sodium and potassium fluoride (calculated as F);

6. from about 0.2 to about 5 points free acid (as herein defined); and

7. at least about 2 and having 8. a total acid/free acid ratio of at least about to l;

9. a zinc/phosphate weight ratio of at least about 0.6 to l; and

10. a nitrate/phosphate weight ratio of at least about 1 to l.

A coating composition and process having these characteristics can be established and maintained in operation to provide high quality zinc phosphate coatings of aluminum, zinc or steel.

The solutions should contain at least 0.7 g./l. of zinc, otherwise the coatings formed, particularly on iron and aluminum, are of low weight and poor quality. There is no real upper limit for the zinc concentration save that dictated by solubility and economic considerations although very high zinc concentrations necessitate a high free acidity and that tends to give poor quality coatings. Generally, however, it is found that a zinc concentration of 20 g./l. is a practical limit and ordinarily it is unlikely that 6 g./l. need be exceeded. As the source of zinc for the bath, or for the preparation of a concentrate for bath make-up there can be conveniently employed zinc oxide or carbonate, through any of the other commonly used zinc derivatives may also be employed.

The solutions of this invention must contain at least I g./l. of phosphate ion, in order to assure that the coating is of an acceptable weight and quality. As for as the coating itself is concerned there is no real upper limit for phosphate, apart from that set by solubility and economic factors. However, the higher the phosphate concentration the greater the tendency for sludge formation, particularly with low quantities of nitrate, and therefore a maximum of 30 g./l. phosphate is recommended, In practice the solutions of this invention operate satisfactorily with from 1 to 20 g./l. (phosphate), and the preferred range is from 1.5 to 10 g./l. The phosphate can be conveniently added to the bath or incorporated into a concentrate, as phosphoric acid.

Although the solutions of this invention contain nitrate, which prevents the formation of tertiary zinc phosphate, nevertheless if the amount of phosphate relative to the amount of zinc is greater than about L5 to l a weight basis-l to l on a molar basis) undesired hydrolysis will occur. Accordingly, the phosphate/zinc weight ratio must be less than at least about 1.0 'to 0.6 L5 to l and preferably about 1 to l.

A coating composition having the above listed characteristics will provide inter alia a means for avoiding sludging by replacing a part of the phosphate with nitrate in the coating bath. Test results given hereinafter indicate that, to maintain a bath acceptably sludge-free, the solutions must contain at points total acid (as herein defined);

least 1 g./l. of nitrate; less is not suflicient to satisfactorily prevent the formation of an unacceptable amount of sludge. It should be pointed out, however, that this g./l. minimum is not an absolute limit since some sludge formation is still encountered, even with somewhat higher amounts of nitrate but at least 1 g./l. will reduce sludge formation sufficiently to permit continuous operation without the periodic stoppages necessitated by sludge build-up in the prior art processes currently in use. While no absolute upper limit for the nitrate concentration can be established, any amount greater than about 30 g./l. would be impractical. A preferred range is from 1.5 to 10 g./l. nitrate. The nitrate is conveniently added to the bath or incorporated into a bath concentrate, as nitric acid. Since the nitrate used in the solutions of this invention replaced part of the phosphate, it will be appreciated that the ratio of nitrate to phosphate plays some part in determining sludge prevention. The test results indicate that the nitrate/phosphate weight ratio must be at least 1 to l (1.5 to l on a molar basis); less nitrate is insufficient to give an acceptable reduction in sludge formation. The ratio can, of course, be higher and a value of 4 to l is recommended as an upper limit; beyond this, good results can still be obtained provided the absolute phosphate concentration is kept above the minimum.

The addition of sodium nitrite as an oxidizing agent provides a means for replenishing the coating bath without incorporating the use of a neutralizing agent such as sodium carbonate. The inherent difficulties encountered in the addition of a neutralizing agent like sodium carbonate, such as for example the tendency toward sludge formation caused by precipitation of zinc near the locality of the addition are thus minimized.

Still another noteworthy advantage gained by using sodium nitrite in the replenishment of the coating bath when used to treat aluminum is that the solution will retain the capacity to coat the surfaces of steel as well. This fact has considerable economic merit. While it is possible to set up and maintain separate industrial scale process lines for the formation of zinc phosphate coatings on the surfaces of aluminum on the one hand, and for the surfaces of iron, steel and zinc on the other hand, it is more economical to formulate a single solution which can be used to form zinc phosphate conversion coatings on metal surfaces irrespective of the particular metal of which they are composed, either wholly or predominantly. MOreover, such solutions are desirable for workpieces which are formed in part of each of the metals zinc, iron and aluminum-for example, truck bodies in the automotive industry. in order for a coating to fulfill these requirements it is preferred that the coating solution contain from 0.006 to l g./l. of nitrite ion. The nitrite ion will oxidize any ferrous iron in the solution (dissolved 011' an iron or iron-containing surface) to fenic iron, which can in turn react with the phosphate to form insoluble ferric phosphate which will precipitate out of solution thereby removing the undesired ferrous iron from the bath. If less than 0.006 g./l. nitrite is used the ferrous iron is not oxidized fast enough, so that the concentration of ferrous ion in the bath rapidly builds up to a level at which the bath becomes inoperative. A preferred minimum nitrite concentration is 0.03 g./l.

The upper limit of l g./l. does not relate to the oxidizing function of the nitrite, but rather to the fact that, in the solution, which is quite acidic, large concentrations of nitrite are unstable, breaking down to form toxic oxides of nitrogen. Accordingly, a low nitrite concentration is desired. A preferred range for the nitrite concentration is from 0.03 to 0.3 g./l. it is, of course, theoretically possible to employ other oxidizers instead of nitrite. Other conventional oxidizers are for example the peroxides, chlorates, bromates and so on. However, the use of such other oxidizers creates problems in bath control and bath replenishment which are not encountered with nitrite and consequently such other oxidizers do not provide the advantageous operational characteristics possessed by the coating baths according to this invention. The nitrite can be conveniently added to the bath. or incorporated into a com centrate as an alkali-metal nitrite, preferably sodium nitrite. Potassium nitrite can also be used or a mixture of sodium and potassium for example in a 1:2 molar ratio can also be used.

The compositions of this invention are as indicated useful for phosphating aluminum as well as iron and zinc and their a1- loys. As noted above, one of the problems encountered in coating aluminum is that following a brief period of operation aluminum ions are dissolved from the surface being treated. The dissolved aluminum poisons the bath significantly hindering, if not substantially preventing, further operation. The addition of fluoride to a zinc phosphate coating bath for treating aluminum is known to cause an etching action on the metal surface and also to form a complex with dissolved aluminum to give a soluble fluoaluminate. As a result the bath in operation will build up a measurable, and detrimental quantity of free aluminum in association with dissolved fluoaluminate. The solubility of the fluoaluminates, however, depends significantly on the cation in the complex and on the crystal form which the precipitating complex can take up as it comes out of solution. Only certain cations can be made available to complex with the fluoaluminate, generally speaking, only the alkali-metal fluorides and bifluorides (and including also the ammonium fluorides) are soluble enough to remain in the solutions as simple fluorides and yet form fluoaluminates which are sufficiently insoluble to provide for removal of dissolved aluminum. Sodium has been particularly preferred for this purpose, not only because sodium fluorides and bifluoride are comparatively inexpensive and readily available, but also because the fluoaluminate fonned can be forced" to precipitate out in the form of cryolite, Na3AlF6, which is acceptably insoluble.

It has now been found that the combined use of sodium and potassium fluorides with a zinc phosphating bath a considerably less soluble fluoaluminate, namely NaKZAlF, (elpasolite) can be formed. The NaK2AlF6 precipitate is removed from solution far more completely and quickly. Since the sodium-potassium ratio in the precipitate is 1:2 it is best if the fluoride is added as a 1 to 2 molar mixture of sodium and potassium fluorides or bifluorides. The bifluorides are preferred though a mixture of fluorides and bifluorides is especially preferred particularly a mixture containing 50 percent normal fluorides and 50 percent bifluorides. in any case it is preferred that the fluoride be added as a 1 to 2 molar mixture of the sodium and potassium salts. The quantity of fluoride employed in the solutions of this invention is from 0.025 to 2.5 g./1. A preferred lower limit is 0.1 g./l. If more than 2.5 g./1. of fluoride is employed then the bath may be in such an acid condition as to cause an undesirable etching effect. For this reason a concentration of 2.5 g./1. is ordinarily not to be exceeded since at this level the etching action is kept to an acceptable minimum while providing rapid complexing and precipitation of any aluminum in the solution. A preferred fluoride concentration maximum is 0.5 g./l. The fluoride concentration may be measured by the standard etching of glass method, or a technique such as is disclosed in US. Pat. No. 2,814,577; 3,129,148; and 3,350,284 may be employed.

The removal of aluminum from solution according to this invention lies in the use of a mixture of sodium and potassium bifluorides as described above. Similarly there can be used a mixture of the nonnal fluorides of sodium and potassium or mixtures of nonnal fluorides and bifluorides. Such mixtures are also used throughout the operation of the bath to maintain the fluoride content within the range specified above. In the case of the bifluoride mixture, the continued addition of such a mixture to a coating solution of the type described when used with aluminum tends to render the solution gradually more acidic, particularly in free acidity." This increased free acidity leads to coatings of an unsuitable sort, or in the extreme case, to no coating at all. On the other hand, in the case of the normal fluoride mixture, continued addition to a coating bath used with aluminum tends to render the solution less acidic, particularly in free acidity," which can likewise lead to poor or nonexisting coatings on aluminum surfaces exposed thereto. Accordingly, replenishment of the fluoride content of the zinc phosphate coating baths of this invention is preferably accomplished by the use of a combination of the fluorides and bifluorides maintaining a KzNa molar ratio of 2:1. The fluorides and bifluorides are preferably present in equal proportion. In this way there is achieved the desired optimum aluminum removal, with the added advantage of having no undesirable efiect of either lowering or raising the free acidity."

The coating solutions of this invention require a free acidity in the range of 0.2 to 5 points. The free acidity of a solution is defined as the number of ml. (points) of N/10 NaOH that will neutralize 10 ml. of the solution, using bromphenol blue (endpoint at about pH4) as an indicator. The free acidity is, as its name suggests, a measure of the free, unassociated hydrogen ion in the solution. If the free acidity of the solutions is less than 0.2 points, then the solutions are not acid enough to ef fect the initial etching of the metal surface before a coating can be formed. Conversely, if the free acidity is above 5.0 points, then the solution is much too acid, so that the surface layer pH can never rise enough for the coating to be formed on the surface. Furthermore, if any coating is formed it would, under such conditions, be dissolved off. The preferred range of free acidity (FA) is from 0.3 to 1.5 points. Generally speaking, the free acidity can be closely correlated with the pH. The overall pl-l should preferably be in the range of about 3.0 to 4.0 (FA of 1.0 down to 0.2) especially in the pH range 3.1 to 3.3 (FA of0.8 down to 0.4)

The solutions of this invention require a total acidity (TA) of at least 4 points. The total acidity of a solution is defined as the number of ml. (points) of N/ 10 NaOl-l required to neutralize 10 ml. of solution, using phenolphthalein (end point about pH 9) as an indicator. The total acidity is, as its name suggests, a measure of the total available hydrogen ion in the solution. In particular it includes the free acid and hydrogen ion formed when dihydrogen phosphate dissociates into monohydrogen phosphate. it is thus, as will be appreciated, a rough measurement of phosphate concentration. 1f the total acidity is less than 4 points the solutions do not contain enough phosphate for the fon'nation of a coating. There is no particular maximum limit set for the total acidity, just as there is none for the phosphate. However, a recommended maximum is 35 points, and a preferred total acidity range is from 6 to 20 points.

The solutions of this invention must have a total acid to free acid ratio of at least 10 to 1. 1f the ratio is less, then regardless of the absolute amounts of either quantity, the solutions will be too acid to provide a coating. There is, however, no particular maximum value for this ratio, although with a very high value the absolute quantity of free acid should be above its minimum. As a practicality though a ratio of greater than 40 to 1 is unnecessary. A preferred range of ratios is between about 12 to 1, and about 20 to l.

Other materials can be added to the solutions of the invention provided, naturally, they are not such as would destroy the ability of the solutions to lay down the required coating. Typical of useful additives are the various accelerating and modifying metal ions, such as nickel'ions. Nickel is particularly useful when coating zinc surfaces (galvanized iron, for example) to promote adhesion of the ultimate paint layer to the coating, and to darken the color of the coating itself. The nickel ion content of the bath solution may vary from about 0.05 to about 1 g./l. The nickel may be conveniently added to the bath. or to a concentrate, as nickel oxide, nickel carbonate or even nickel nitrate.

The solutions of this invention are conveniently operated at a temperature of from about 45 C. (113 F.) to about 60 F. F.), at which temperatures good coatings can be obtained with contact times of between about 15 seconds to 3 minutes. Temperatures lower than 45 C. can of course be employed though in such instances reaction time is really too long. Above 60 C. there is an increasing tendency for zinc to precipitate out as tertiary zinc phosphate (at 65 C. this becomes noticeable, and becomes even more evident as the temperature increases). The time for which any surface is allowed to remain in contact with the solution depends primarily upon the solution temperature and on the weight of coating required. Generally speaking, the more acceptable coatings are obtained at lower temperatures with longer contact times.

The bath solutions of this invention, when in operation, sufi'er depletion caused by, for example, actual use of chemicals to form the coating, the formation of sludge, i.e. iron phosphate and by drag-out on the surface being coated. Accordingly, it is periodically necessary to replenish the bath by adding the required chemicals in amounts appropriate to counteract depletion. The replenishment procedure for the solutions of this invention is extremely simple and easy to carry out. This in itself provides a clear advantage over prior art solutions, which require a complex replenishment procedure. The solutions of this invention can be replenished with three liquid concentrates regardless of the metal being treated and furthermore the addition of each of these liquid concentrates can be closely controlled by automated techniques. These three materials are; l) the makeup concentrate (sufficient of which is added to restore the total acidity and free acidity to the desired values); 2) nitrite ion, conveniently added as sodium nitrite (the amount required can be determined by a standard titration against permanganate); and 3) sufficient fluoride to restore the fluoride content to within the desired range. As specified by the ranges given above and as further indicated by the evaluations to be given with examples hereinafter, monitoring and replenishing of the bath are necessary to keep the solution in good coating condition.

The makeup concentrate (which, as its name implies, is used to make up the original bath) contains zinc, phosphate, nitrate and nickel ions in the appropriate proportions.

The nitrite concentrate contains only nitrite (usually as the sodium salt) conveniently at a concentration of about 2 lbs/gal.

The fluoride mixture concentrate contains a mixture of sodium and potassium fluorides and sodium and potassium bifluorides. A typical fluoride concentrate contains about 0.7 1b./gal. sodium and potassium fluorides and bifluorides. The sodium and potassium are preferably present in a molar ratio of about 1 to 2. The relative amounts of fluoride and bifluoride is not critical though for best results and being adjusted in order to provide for the suitable pH and free acidity in the bath the fluorides and bifluorides are usually employed in about equal amounts. Such mixtures of fluorides and bifluorides of potassium and sodium can be prepared as a stable, nonlumping powder or as liquid concentrates in which the fluoride mixture is dissolved in water, such powder or concentrate is useful for makeup and replenishment of phosphating baths constitutes a part of this invention. The powder formed is substantially nonhygroscopic and has a long shelf life.

if it is desired to coat aluminum, steel or galvanized, either alone or in combination a bath set up and maintained according to this invention can be used for any and all metal combinations, provided that into the basic phosphating solution, containing ions of zinc, phosphate, nickel, nitrate and nitrite,

there is also incorporated the additives indicated below.

Additives to Basic Phosphating Metals to be Treated Solution aluminum, steel and zinc NaNO, fluoride mixture aluminum and steel NaNO, fluoride mixture aluminum and zinc NaNO, fluoride mixture steel and zinc NLINO steel NaNO, aluminum NaNO fluoride mixture zinc none required Fluoride mixture can of course be added in all instances.

. cession of aluminum surfaces The metal surfaces to be phosphatized should first be cleaned, and it is preferred to clean the surfaces with an alkaline rinse. After the phosphatizing process, the coated surface is preferably given an after-rinse with a hexavalent chromium or other final rinse, to improve its corrosion resistance.

In a preferred sequence of operations the metal surface to be coated is precleaned with a conventional alkaline rinse at elevated temperatures, followed by a water rinse. The conversion coating is then formed, using a solution according to this invention and the coated surface is given a water rinse and a conventional after-rinse. The precleaning and after-rinse solutions can be of the type ordinarily employed in the art in connection with the formation of chemical conversion coatings on iron, zinc or aluminum.

- The novel processes and compositions of this invention will be more fully understood from the examples which follow. These examples are merely illustrative of the invention and are not to be construed as limitative thereof.

EXAMPLE 1 A general purpose solution designated as solution A for preparing a zinc phosphate coating bath according to the prior art was prepared as follows:

Solution A Contents Amount in Grams ZnO 143 No, 233 H,Po,-. 75% 205 Water to a specific gravity of 1.48 at 60 F.

grams of the above composition was diluted to 1 liter and adjusted in acidity so that 3.5 to 4.0 cc. of N/l0 NaOH was required to neutralize 10 cc. of the solution using bromphenol blue as indicator (free acidity") and from 38 to 40 ml. of the hydroxide using phenolphthalein as indicator total acidity). A solution made up in this way (solution B) was sprayed upon the metal surface (alternatively articles to be coated may be dipped into the solution).

According to the art, such a solution can be used to treat a succession of aluminum articles or its alloys, depositing a zinc phosphate conversion coating on the surfaces thereof, provided that there is incorporated into the bath sufficient fluoride salt (say sodium bifluoride) to provide a concentration of 0.2 to 2 g./l. of fluoride. The purpose of the fluoride salt is to combine with the free aluminum dissolved during the coating process and to remove it from the solution as Na AlF (cryolite). In order to prevent the fluoride concentration in the solution from becoming depleted the fluoride is replenished with a replenishing solution prepared according to the prior art comprising sodium bifluoride and a neutralizing agent, such as sodium carbonate. A convenient means for measuring the content of fluoride ion is by the simple technique disclosed in U.S. Pat. No. 3,129,148 or more preferably by the more sophisticated techniques disclosed in Pat. Nos. 3,329,587 or 3,350,284.

Solution C was prepared by adding 0.5 g./l. fluoride to solution B prepared as above. The solution was used to coat a suc- (3005 alloy) by spray impingement for a period of one minute each. The temperature of the solution was maintained at F. during operation. During spraying the coating solution C was replenished with,

1. Sufficient solution A to maintain the total acidity pointage at or slightly below its initial value,

2. sodium bifluoride to maintain the fluoride ion concentration at its initial level, as indicated by the device described in U.S. Pat. No. 3,350,824 and I 3. sodium carbonate, as a neutralizing agent to maintain the free acidity and or slightly above its initial level.

Results were as follows:

Fluoride activity, Loading, (micro- Run ti/liter FA TA Ratio amps) Coating I 4.1 38 9.3 220 Good, uniform.

1 4.8 37 7.7 200 Thin, Streaky. 2 4.5 42 9.3 225 Very poor. 3 7.0 48 6.9 225 None. 5 3.6 36 10.0 240 Good.

II 0 4.0 39 9.7 250 Good.

1 4.2 38 9.1 250 Fair. 4 4.3 .37 8.6 210 Thin. 4.0 37 9.4 240 Poor.

1 Thefluoride activity is measured in microamps on a fluoride activity measuring device of the type disclosed in U.S. Pat. 3,350,824.

2 FA lowered with sodium carbonate.

From the above results it can be seen that increased loading results in increased free acidity. It can be further seen that extended. bath loading and replenishment of a solution such as C above results in the formation of poor coatings, 'even though the fluoride ion content (measured as fluoride activity) and the total acidity" pointage were maintained at their initial values. Analysis of the replenished bath, and of the sludge which formed during the loading operation, demonstrated an almost total loss of zinc ion from the coating bath which was removed from the solution as sludge.

EXAMPLE 2 Zinc phosphate solution concentrates where prepared having various Zn:PO :NO ratios as follows:

heavy sludging. The solutions H and .1, containing higher concentrations of phosphate, exhibit greater sludging and more rapid loss of zinc than solutions K and L, which contain lower amounts of phosphate.

Further loading baths H through L with aluminum panels, and replenishment of each bath with its respective concentrate (solutions D through G) and with sodium bifluoride and sodium carbonate, resulted in a loss of coating ability, first in solution H, then in solution J as more and more zinc was lost from solution, probably in the form of insoluble tertiary zinc phosphate. Solutions K and L maintained their coating ability, although some slight zinc loss was noted.

EXAMPLE 3 A solution identical to coating solution K, heretofore mentioned, was prepared, and a 3 inch X 4 inch soft glass slide was immersed therein. The weight losses of the glass slide at 15, and 60 minutes was noted to be 0.4, 2.2. and 3.8 mg., respec tively. Sufficient boric acid was then added to solution K to complex the fluoride therein, (solution M) substantially no further weight loss (0.3 mg.) was noted on 3 inch X 4 inch soft glass slides immersed in the solution for periods of one hour or more. Further evidence of the presence of free fluoride in the case of coating solution K on the one hand. and the absence of free fluoride in the case of coating somtinn M on the other hand, can be shown by measurements with the device described in US. Pat. No. 3,350,284. Solution K generated 90 microamps, while solution M generated less than 10 microamps by the same technique indicating a pronounced Gram Gram Grant 30 difference in fluoride activity and consequently in fluoride Solution Moles Moles Moles o e tratio Concentrate Zn PO NO,

EXAMPLE 4 o 2 4 l r E 2 a z The following zinc phosphate coating solution base was F 2 2 3 re ared: o 2 l 4 p p Gals. Lbs '1 by WI Z- ox'de 1.359 11.74 A 2% percent (volume/volume) bath solution was prepared 40 zi :idc 0098 0'84 from each concentrate. 15% Phosphate acid 0. we 2.l8l 18.84 25 ml. of each of the solution concentrates were diluted, 0172 Base 2 0.001 0.0a: 0.72 respectively and individually, to l liter and there was added 75% phusphom and 0.5 g./l. of fluoride as a mixture of potassium and sodium such Dissolved iron that the sodiumzpotassium ratio was 1:2 The resulting solu- Water 0.574 4.734 .33 tions formed from concentrates D, E, F and G, above, were identified respectively as coating solutions H, J, K and L hav- "'8 the followmg charactensucs- A zinc phosphate concentrate was prepared by dissolving zinc oxide and nickel oxide in a mixture of phosphoric and Coming FA TA nitric acid. After the mix cleared (Base Z) was added and the Solution (Pointage) (Pointage) g./l. mixture allowed to cool.

There were also prepared solutions of normal fluorides and 8-38 :31 bifluorides of sodium and potassium designated N, O, P and O K 0.40 10.3 3.4 as follows: L 0.45 9.9 3.4

Solution .0. NuFHF g./l. KF-HF Na:K

Solutions H through L were applied to a series of aluminum 8 3 2 3 5f? panels. A total of 2 ft. of metal surface per liter coating solup 349 m tion was treated. The results obtained and the changes in bath o 2 l 5.2 1:2 composition are shown below.

Initial, g./1. Loaded, g./l. Change, g./l. Coating solution FA TA Zn FA TA Zn FA TA Zn Bath lippvarancv l4. 7 3.3 1. 2 13.1 2. 7 +0.3 0.6 O. 6 Immediate cloudiness. 13. 4 3. 4 1. 0 l2. 6 2. 7 +0. 2 O. 8 O. 7 loudim'ss int-rousing wit h time. 10. 3 3. 4 0. 40 10.1 3.0 0 O. 2 0. 4 No initial sludging.

As demonstrated by the above results, reducing the phosphate concentration greatly reduced the zinc loss, pointage loss, and free acid increase after loading, while preventing To each solution N. O. P and Q was added a stoichiometric amount of aluminum (as the nitrate). The resulting complex fluoaluminate precipitates were dried and weighed.

COMPARISON OF A] PRECIPITATION Expected of Al Residual Solution Constituency Product Precipitated Aluminum N NaF HF100% Na AlF 7| 2590 p.p.m. KF-HF- 100% K A IF, 71 2580 P NaF-HF50% K,NaAlF 88 1080 KF-HF 50% +Na,A|F, O NaFHF-Vs lemme, 97 I90 KF-HF% These results show that molar ratios of 2K:1Na yielded a far greater removal of aluminum from solution and resulted in a lower residual aluminum concentration than the other proportions tested.

To a zinc phosphate solution formed by dilution of the zinc phosphate coating solution base prepared as above there was added 0.1 mole of the fluoride, solutions N, O, P and Q, respectively, the following precipitates were obtained upon treatment with a stoichiometric amount of aluminum nitrate:

As illustrated by the results tabulated above the compositions containing mixtures of potassium and sodium, such that the major precipitate formed was elpasolite, K NaAlF produced the densest, fastest-settling sludge. It can be further seen that the most complete removal of dissolved aluminum from solution was effected by a mixture of bifluoride salts such that the molar ratio of K to Na is 2:1. in addition, the complex aluminum fluoride precipitate which was formed in the case of solutions Q was of a better nature for the purposes of this invention than those of solutions N, O, or P, i.e. the precipitate was dense, settled quickly, and was not easily disturbed by slight agitation of the solution but was easily removed from the container when desired. There qualities are beneficial when the process is adapted for commercial purposes, where it is desirable for the sludge which is formed to settle quickly to the bottom of the vessel and to remain there until removed by mechanical means.

EXAMPLE The mixture of fluorides and bifluorides, hereinafter referred to as mixture R is such that the ratio of fluoride and bifluorides is 1:1; and the ratio of potassium to sodium is 2: l. Mixture R is prepared as follows:

Lbs. k by Wt.

Potassium bifluoride, anhydrous 0.415 41.5 Sodium bifluoride anhydrous 0.165 I65 Potassium fluoride. anhydrous 0.309 30.9 0.! l l 1 LI Sodium fluoride, anhydrous was added sufficient of mixture R above to form a concentration of 0.5 g./l. of fluoride ion, and sodium nitrite to a concentration of 0.12 g./l. of nitrite. The solution was adjusted with sodium hydroxide to a free acidity" and total acidity" of 0.6 and 10 points, respectively. This solution is hereinafter referred to as solution 8.

A succession of cleaned aluminum panels (3003 alloy) was subjected to impingement with solution S, heated to F. by spray for a period of one minute each. The coating solution S was maintained in total acidity and fluoride concentration in a manner similar to that described in example 1, using solution F, heretofore described, to maintain the total acidity and mixture R to maintain the fluoride content, conveniently measured by the device described in US Pat. No. 3,350,284. The nitrite content was determined by a titration against 0.042 Normal permanganate, 25 cc. of solution S, requiring 3.1 ml. of permanganate, and replenished to maintain this concentration of nitrite, or slightly less, with a solution of sodium nitrite. Continuous evolution of volatile nitrous acid was detectable. The solution S was able to successfully coat a long succession of aluminum panels, the essential parameters of the test being as follows:

pH RANGE OF OPERATING BATH Operating range-3.00 to 3.40

Preferred range-3.10 to 3.30

pHs below 3.00 resulted in thin, sparse, discontinuous coatings.

pH's above 3.40 caused an almost complete loss of free acidity and insolublize the zinc.

ETCH RATE ON GLASS EFFECT OF HIGH NITRAIE P0 'IDttti Coating FA TA Ratio g./l. N0 g./l. weight. Appearance 0. 6 10 16. 7 5 5 320 No change. 0- 6 l0 l6. 7 5 10 241i Do. 0. (i 10 16. 7 5 15 274 D0.

High nitrate concentrations did not appear to significantly alter the weight or appearance of the coatings on aluminum.

Zn-PO NO RATIO Observed Rungc Observed Range Component gl/l. mol/l. Ratio Zn 1.3-4.0 0.02-0.06 1 PO 2.7-7.2 0.03-0.08 05-! NO, 4-15 or 0.06-0.03 l-l5 Lower limits were established as those which would insure coatability. Upper limits were established in order to prevent hydrolysis and for economy of operation.

EXAMPLE 6 coating weight was determined by measuring the weight loss of a sample specimen after chemically removing the coating. The coated panels were subjected to the following series of tests:

1. adhesion of a brittle, acrylic-based paint applied as a onecoat system,

2. corrosion resistance under two different alkyd/paint systems, similar to those used in the automotive industry, subjected to a salt fog in accordance with ASTM Spec. 8-1 17-64 for 250 and 336 hours. respectively.

By way of comparison, there were also subjected to these tests a set of untreated painted aluminum panels and a set of aluminum panels coated by an illustrative prior art process, a chromate treatment for aluminum only utilizing a coating bath composition prepared as follows:

Lbs.

Base W 0.33 Comprised of: potassium fluozirconate 18.8% by wt. potassium fluoborate 6|.38'X by wt. sodium fluoride 20.44% by wt. Potassium ferricyanide 0.!3 Chromic acid 0.54

periods of high humidity. All chemicals should be tree-flowing powders or granules.

Also included for comparison were sets of untreated painted zinc and steel panels and zinc and steel panels treated by illustrative prior art treatments for zinc and steel only. The

zinc and steel treatment composition was prepared as follows:

Half the formula quantity of zinc oxide and all the nickel oxide were slurried in hot water and mixed thoroughly. Zinc oxide was added until the slurry began to thicken. The phosphoric and nitric acids were added and the remaining zinc oxide was added as rapidly as possible. When the mix was clear Base Z was added with agitation and the batch allowed to cool. When cool, the fluosilicic and hydrofluoric acids were added and mixed thoroughly.

FA 1 ml. concentration requires l.95-2.05 ml. 1.0N NaOH TA 1 ml. concentrate requires 8.80-8.90 ml. LON NaOH The comparative results on treatment of aluminum and CrO g Kama). steel panels with solution S IS shown below. '',2'*" Good coatings were obtained during extended bath loading 4 5:38.2 2:15 2; under varymg concentrations of 2106 and phosphate. Ade- Through BO-fllcih screen 5% 18% quate coatings were maintained at phosphate concentrations of 3-4 g/l.

BATH LOADING WlTJI ALUMINUM ()NLY Rcplenish- Zn. Flupridv P01, Ftfi/l. merit rate FA TA Ratio g./l. activity (3W g./l. (oattng Run "A" 0.5 16.8 3.1 05 4.7 Good. 8.6 14.4 2.3 108 Do. 8.4 12.0 2.0 8.0 110. 8.0 0.0 2. 4 05 110. 0.0 10.0 2.0 4.5 Do. 7.2 12.0 1.8 110 170 3.4 Do.

N02, Zn. Fluoride P04, FtJ/l FA TA Ratio g./l. g./l. activity g./l. CW oating R By! Base W and potassium ferricyanide were mixed for about 5 minutes and the granulate chromic acid was added and the entire composition mixed for an additional 5 minutes. Exposure of the chemicals to the atmosphere should be kept to a minimum and manufacturing should be avoided during Phosphate depletion was faster during loading of steel because of FePO, sludge formation. Good coatings on steel, aluminum, and sample galvanized were maintained at as low a pointage as seven, which is well below the data sheet recommendation.

BATH LOADING WITH STEEL ANI) ALUMINUM N0'1E.- Typical coating weights: Al, 164; Stool. 357; (lnlv., 335.

UNDER-POINT PERFORMANCE RESULTS Duracron adhesion --I--- I Ford 1 50 111. lb. Conical Tape Pencil Salt (iilsouite 1 impact mandrel Knife cross hatch hardness spray salt spray $4 slight flake 2% slight crack Satisfactory. No loss 2H 10 5 o 2%slightcrack... o U do ill 9.5 slight crack 2" very slight crack. .-do do 211 10 7.0 o d 2H 10 6.0 r 1% very slight crack .do H 10 .l. 5 do. very slight crack..." do. -do 11 10 1|. 5 Al-alodine do. very slight crack .do i so II 10 l 5 Al-untreated Total loss 4" total loss Unsatisfactoi y Heavy loss 11B 10 SJ. 5 Al do. d0 .do Total l0ss- B 10 El- 5 1 Salt spray ratings according to PP346802. 2 Metal ruptured from force of impact.

EXAMPLE 7 Spray Application Good coatings were obtained on aluminum steel and galvanized surfaces when the coating solution was applied by spray application according to the following procedure.

Coating Bath Makeup For each 100 gallons of bath then was added to the water, in sequence, with stirring:

Zinc phosphate coating base 2.5 gal. (29 lbs.)

Composed of:

Gals. Lbs. by wt.

zinc oxide 1.359 ".74 nickel oxide (75% Ni) 0.098 0.84 75% phosphate acid 0. I 66 2.181 18.84 38 Be nitric acid 0.272 3.07] 26.53 Base 2 (as in Ex. 4) 0.007 0.083 0.72 Water 0.574 4.784 L33 Caustic soda solution 0.3 gal. NaNO solution (2 lbs/gal. 0.25 to 0.74 pints Fluoride concentrate l gal.

Composed of:

Lbs. '1 by wt.

potassium bifluoride, 0.415 4L5 anhydrous sodium bifluoridc. 0.165 16.5

anhydrous potassium fluoride. 0.309 30.9

anhydrous sodium fluoride, OJ 1 I l L! anhydrous The caustic soda solution was prepared by dissolving 2 lbs. of caustic soda per gallon of water.

Normal Operating Conditions fluoride activity reading 50 to 250 microamps Process Sequence Operation No. l Clean Operation No. 2 Rinse Operation No. 3 Coat Operation No. 4 Rinse Operation No. 5 Aciduluted Acidulated rinse The work, after processing and drying was ready to be painted.

Surface Preparation The work was cleaned using a suitable alkaline cleaner. If the work is heavily soiled and additional cleaning power is required a detergent cleaner additive may be added to the alkaline cleaning bath.

The work, after cleaning, was rinsed with water and the rinse was continuously overflowed to avoid contamination. Maintenance of the Bath The bath was manually controlled in plant by a Pointage, A Free Acid, :1 Nitrate Titration and Fluoride Reading using a fluoride activity meter such as the commercially available Llneguard Meter No. 101A. The other components of the bath were monitored either electronically or chemically. chemical monitoring can be carried out as follows:

l. Pointage (Total Acid) Titration:

a. Pipette a 10 ml. sample of the zinc phosphate coating base solution, made up as described above into a beaker.

b. Add 6 to l0 drops phenolphthalein.

c. Fill the automatic burette to the zero mark with 0.l N. sodium hydroxide.

d. While stirring the sample, slowly run in O.l N sodium hydroxide until a pink color is obtained.

e. Record the number of milliliters of sodium hydroxide used as the Pointage (Total Acid) Titration.

Replenishment Add 1 quart (3 lb.) of bath makeup solution per gallons of bath for each point lacking. The bath should be kept within 2 points of the specified pointage.

2. Free Acid Titration a. Pipette a 10 ml. sample of the bath solution into a beaker.

b. Add 2 to 3 drops of Brom Phenol Blue.

0. Fill the automatic burette to the zero mark with 0.1 N. sodium hydroxide.

d. While stirring the sample, slowly run in the sodium hydroxide from the automatic burette until the color changes from yellow to light blue.

e. Record the number of milliliters of sodium hydroxide used as the Free Acid Titration. 3. Acid Ratio Determination Determine the acid ratio by'dividing the pointage (Tptal Acid) by the Free Acid. For optimum results, the acid ratio should not fall below 12. if the acid ratio is too high, but pointage is normal and marginal coatings are produced, a portion of the bath is dumped and sufficient water and bath makeup solution are added to restore the bath to proper pointage. If the acid ratio is low, but pointage is nonnal and marginal coatings are produced, the nitrite content is increased. 4. Nitrite Test and Titration a. Dip a strip of ferrous iron test paper No. 2005 into a sample of the bath. If a blood-red color develops, ferrous iron is in the bath, and there is a b. of nitrite solution. The bath is replenished with the starting concentration of nitrite solution and the test is repeated.

b. Pipette a 25 ml. sample of the zinc phosphate coating base solution into a beaker.

c. Add 5 to 10 ml. of 50 percent C. P. Sulfuric Acid.

d. Fill the automatic burette to the zero mark with 0.042 N. potassium permangante.

e. While stirring the sample, slowly run in the potassium permanganate until a pink color persists for at least l seconds.

f. Record the number of milliliters of permanganate used as the nitrite titration.

Replenishment Add approximately 3 fl. oz. of NaNO solution per 100 gallons of bath for each milliliter lacking. Whenever a portion of the bath is discarded or lost by sludge removal or leakage, the volume should be restored with the same proportion of chemicals and water as used in the original bath. Bath strength is best maintained using a volume-regulating feed pump.

After the bath has been made up and, if needed, adjusted to particular processing line requirements, the fluoride content (active fluoride component) was determined by using Lineguard Meter No. lOlA (described in US. Pat. No. 3,350,284).

The Lineguard Meter No. l0lA reading showing the initial fluoride activity in the bath was recorded and served as basis for comparison with subsequent fluoride acidity readings/concentration to determine the amount of fluoride replenisher needed for continued operation.

Operational Recommendations 1. The makeup of the coating bath as given above is recommended for the simultaneous coating of aluminum, steel, and galvanized. Where one or more of these metals is to be omitted from the processing sequence, the following chart will serve as a guide-line for the recommended compositing starting with the zinc phosphate coating base and caustic soda solution of the above coating bath makeup.

Additional component for inclusion with zinc phosphate coating bath Flouridc concentrate (alkali nitrito.

Metals to be treated Aluminum, steel and Aluminum and steel Aluminum and galvanized. Steel and galvanized..."

galvanized.

Do. Alkali nitrite.

After coating treatment, the treated metal is given a water rinse and an acidulated rinse.

Coated materials coming from the final acidulated rinse should be dried as soon as possible in an indirectly fired oven or by other means which will not contaminate the metal with fumes, oil, or partially burnt gases. In many cases, heavyguage metal will retain enough heat to dry completely and rapidly without using an oven.

The compositions and processes of this invention are readily adapted to conventional power-spray processing equipment. The equipment for the coating stage is ordinarily constructed of stainless steel. All other stages may be constructed of mild steel. All heated tanks are preferably equipped with steam plate coils and side heating (preferred for a more even temperature distribution) or other heat sources capable of rapidly heating the bath to the specified temperature.

We claim:

1. A phosphate coating solution capable of producing paint receiving coatings on surfaces of iron, zinc and aluminum wherein the coating solution is an aqueous acidic solution consisting essentially of water, at least about 0.7 gram per liter of zinc, at least about 1.0 gram per liter of phosphate (calculated as P0 at least about 1.0 gram per liter of nitrate (calculated as N0 between about 0.006 and about l.0 gram per liter of nitrite (calculated as N0 between about 0.025 and about 2.5 grams per liter of fluoride (calculated as F), at least about 0.005 gram per liter of sodium, and at least about 0.017 gram per liter of potassium; such that the ratio of nitrate to phosphate is at least about l:l and the ratio of zinc to phosphate is greater than lzl and wherein the potassium to sodium molar ratio is about 2: l and the fluoride is present as a result of adding a mixture of fluorides and bifluorides in a ratio of about 1:1.

2. A coating solution according to claim 1 wherein the free acid content is between about 0.2 and 5 points and the total acid content is at least about 2 points such that the total acid to free acid ratio is at least 10:1.

3. A stable, nonlumping, substantially nonhygroscopic powder consisting essentially of anhydrous NaF and about I part by weight of anhydrous NaF-HF per part of NaF and about 2 parts by weight of each of anhydrous KF and anhydrous KF'HF per part of NaF.

4. A coating solution according to claim 1 wherein the potassium to sodium molar ratio and the fluoride concentrations are such that the addition of dissolved aluminum will cause the precipitation of K NaAlF 5. A method for producing zinc phosphate coatings on a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least about I gram per liter of phosphate ion (calculated as PO. (c) at least about 1 gram per liter of nitrate ion (calculated as N0 (d) between about 0.006 gram per liter and about 1 gram per liter of nitrite ion (calculated as N0 (e) between about 0.025 gram per liter and about 2.5 grams per liter of fluoride added as a combination of fluorides and bufluorides of sodium and potassium, such that the molar ratio of potassium to sodium is about 2: l and the ratio of fluoride to bifluoride is about 1:1.

6. A method according to claim 5 wherein the metal surface being treated is aluminum or an alloy of aluminum and the solution is maintained substantially free of dissolved aluminum by maintaining the solution content of fluorides and bifluorides of sodium and potassium such that dissolved aluminum is precipitated from solution as K NaAlF 7. A method according to claim 2 wherein the nitrate to phosphate ratio is maintained at about lzl or greater; the ratio of zinc to phosphate is greater than I: l; and the nitrite content is maintained between about 0.006 gram per liter and about 1.0 gram per liter by the addition of alkali metal nitrite; with the free acid content maintained between about 0.2 and about 5 points and the total acid content maintained at greater than about 2 points such that the total acid to free acid ratio is at least about lOzl. 

2. A coating solution according to claim 1 wherein the free acid content is between about 0.2 and 5 points and the total acid content is at least about 2 points such that the total acid to free acid ratio is at least 10:1.
 3. A stable, nonlumping, substantially nonhygroscopic powder consisting essentially of anhydrous NaF and about 1 part by weight of anhydrous NaF.HF per part of NaF and about 2 parts by weight of each of anhydrous KF and anhydrous KF.HF per part of NaF.
 4. A coating sOlution according to claim 1 wherein the potassium to sodium molar ratio and the fluoride concentrations are such that the addition of dissolved aluminum will cause the precipitation of K2NaAlF6.
 5. A method for producing zinc phosphate coatings on a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least about 1 gram per liter of phosphate ion (calculated as PO4), (c) at least about 1 gram per liter of nitrate ion (calculated as NO3), (d) between about 0.006 gram per liter and about 1 gram per liter of nitrite ion (calculated as NO2), (e) between about 0.025 gram per liter and about 2.5 grams per liter of fluoride added as a combination of fluorides and bufluorides of sodium and potassium, such that the molar ratio of potassium to sodium is about 2:1 and the ratio of fluoride to bifluoride is about 1:1.
 6. A method according to claim 5 wherein the metal surface being treated is aluminum or an alloy of aluminum and the solution is maintained substantially free of dissolved aluminum by maintaining the solution content of fluorides and bifluorides of sodium and potassium such that dissolved aluminum is precipitated from solution as K2NaAlF6.
 7. A method according to claim 2 wherein the nitrate to phosphate ratio is maintained at about 1:1 or greater; the ratio of zinc to phosphate is greater than 1:1; and the nitrite content is maintained between about 0.006 gram per liter and about 1.0 gram per liter by the addition of alkali metal nitrite; with the free acid content maintained between about 0.2 and about 5 points and the total acid content maintained at greater than about 2 points such that the total acid to free acid ratio is at least about 10:1. 